To furrow the ground is to cut long, narrow grooves in the ground, so that water can flow in, and so that you can plant things in nice tidy rows. Check out this farmer furrowing the field: adding furrows to it, or long trenches.
Anything that looks like those long, narrow wrinkles in the dirt can also be called furrows. In fact, on our foreheads, we tend to get furrows: long wrinkles. Especially when we make a serious face. Squish your eyebrows downwards, like you're angry or worried, and you're furrowing your brow.
We see that phrase a lot in stories. "He furrowed his brow, feeling suspicious." "With a furrowed brow, she frowned at us." "His brow was furrowed in anger."
Remember: to furrow your forehead is to wrinkle it because your face is showing anger, sadness, or some other serious emotion. And furrows are long grooves or trenches.
Idea 1: "My brow furrows as I worry about (something difficult I have to do soon)."
Idea 2: "(Someone) looks at me, (his or her) brow furrowed, and asks, 'What if (something really bad happens)?'"
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